Free way to improve Pro Logic I

[admin_exported]

It seems I'm not alone in still using Pro Logic I to liven up stereo (analogue) TV, though I wish I knew how it works - some progs are so improved by it (eg Dr Who, Spooks and movies) that surely they must be encoded in some way? And how does it send the crowd to the back of the room on live concert recordings made 40 years ago if I play (old, not remastered) CDs through it?

Anyway, I recently extended the cables to the rear speakers so that I could drag them out into the room and sit directly between them, but if anything it sounded worse: the 'surround' sound, being mono, seemed to be coming from inside my head and sounded very dull and unnatural. It struck me that this too-focused effect could be "corrected" by wiring the speakers out of phase (ie swap the connections on one of the rear speakers.) It worked!!! I was suddenly truly "surrounded" by sound. OK it's doubtless not as good as 5.1, but honestly, it's a huge improvement and absolutely free and reversible. Give it a go, even though it must go against your every audiophile instinct.

I'm using a Thomsom DPL2000 amp which you probably won't be impressed by, but I found it new for £40 in a closing down sale and simply couldn't resist!

 

[Anonymous]

What you're describing is called 'de-correlation'. It's been a part of the THX post-processing system for over a decade (sorry if that spoils your excitement, by the way). Essentially, it's an attempt to trick your ears into perceiving more spaciousness in the soundfield than there actually is.
To do this, de-correlation attempts to differentiate sounds arriving from the left-rear speaker from those arriving from the right-rear speaker - which is difficult to do, of course, because as you point out, Dolby Pro-Logic only offers a single mono surround channel. One way to achieve this is to run one of the rear speakers 180 degrees out of phase from the other (in the case of THX kit, this is done by the processing circuit in the AV amp/processor/receiver). One downside is that this also tends to produce a bass cancellation effect at the back of the room. That's partially why THX-certified rear speakers can be on the large side.
For more, see www.thx.com/technologies/decorrel.html
Finally: Dolby Pro-Logic processing tends to deliver more exciting, 'effects-driven' listening when applied to content encoded in Dolby Surround sound, as you've correctly surmised. In fact, many UK TV broadcasts carry that encoding, although it's rare to see it advertised (in my experience only American shows, such as The Simpsons, make a big deal of it). Of course, Pro-Logic can also deliver surround sound from two-channel content, but in this instance, the processor is having to do the surround-steering job 'on the hoof'. When processing encoded Dolby Surround content, it's merely doing what it's told.

 

[Anonymous]

Doesn't spoil my excitement one jot! I'm still delighted with my free 'upgrade'. THX could have been around since the dawn of time, but how does it benefit me? And lack of bass is not a problem as it's more important that I stay on speaking terms with my neighbour.

I didn't think I'd discovered anything new; I'm sure many of us have accidentally wired speakers out of phase and noticed the effect but I'd never thought it could actually be beneficial. I did it with a two speaker set up, and mono radio was very odd and nebulous and this is what made me try it for the surround system. I suppose the effects options ('stadium' etc) are similar, but they are so over the top as to be utterly pointless - on my amp at least.

Sorry if my 'bright idea' turned out to be general knowledge, but it is so effective I thought it worth sharing with those not blessed with bottomless pockets and wanting to make the most of existing equipment. (Why exactly do you need a "processing cicuit in the AV amp/processor/receiver" to put the speakers out of phase? - don't answer that.) Perhaps the clued up readers of What Hi-Fi weren't the right audience, but I was only trying to be helpful.

BTW, A Touch Of Frost has had a surround sound notice at its start for many, many years. (I'm probably the only contributor to these forums to have never watched an episode of The Simpsons!)